CA1313032C

CA1313032C - Method of producing an oxide superconductor without sheath and an oxide superconductor produced by the method

Info

Publication number: CA1313032C
Application number: CA000579107A
Authority: CA
Inventors: Masaru Sugimoto; Nobuyuki Sadakata; Shin'ya Aoki; Toshio Usui; Mikio Nakagawa; Taichi Yamaguchi; Osamu Kohno; Yoshimitsu Ikeno; Atsushi Kume; Kenji Goto
Original assignee: Fujikura Ltd
Current assignee: Fujikura Ltd
Priority date: 1987-10-02
Filing date: 1988-10-03
Publication date: 1993-01-26
Anticipated expiration: 2010-01-26
Also published as: EP0311337A3; EP0310453B2; EP0310453B1; DE3880947T3; EP0310453A3; EP0310453A2; CA1313031C; EP0311337B1; EP0311337A2; DE3880947T2; DE3880947D1

Abstract

Abstract of the Disclosure A method of producing a superconductor including a superconductive oxide. The superconductive oxide is represented by the formula AxByCzD7-.delta.
provided that the A is at least one selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, the B is at least one selected from the group consisting of Be, Sr, Mg, Ca, Ba and Ra, the C includes Cu, the D includes 0, about 0.1 < x < about 2.0, about 1 < y <
about 3, about 1 < z < about 3, 0 < .delta. < 5, or by the formula AxByCazCuiOj provided that the A includes Bi or Tl, the B includes Sr or Ba, about 1 < x < about 3, about 1 < y < about 3, about 0 < z S about 3 and about O-< i-~ about 9. A filling material is charged into a metallic pipe for forming a preform. The filling material is at least material selected from the group consisting of a starting material powder of the superconductor, a powder of the superconductor and a compact made of the starting material powder and/or the superconductive powder. The preform is reduced in cross-sectional area for orming a composite including a core, made of the at least one filling material, and a metallic sheath covering the core. The metallic sheath of the composite is removed for exposing the core. The exposed core is heat treated for producing the superconductive oxide.

Description

131303~

METHOD OF PRODUCING AN OXIDE SUPERCONDUCTOR WITHOUT A
SHEATH AND AN OXIDE SUPERCONDUCTOR PRODUCED BY THE METHOD

Backyround o~ the Invention Intense ePforts in research and development are directed ~ toward superconductive oxides for practical use, for ; example, magnet coils of the nuclear magnetic resonance imaging apparatus, magnet coils of the particle accelerator, the power transmission line and a like use. The present invention relates to a method of producing a superconductor which exhibits excellent superconductivity, particularly high critical current density as compared to the superconductor produced by the prior art method. Some of the inventors proposed oxide superconductors in the following applications as joint inventors: U.S. Patent No.
4,885,273 entitled "Method of producing a superconducting wire and a superconducting wire produced according to the same"; European Patent Application Publication No.
o 283 313.A3 entitled "Method of producing oxide super-- 20 conducting wire and oxide superconducting wire produced by this method"; European Patent Application Publication No.
0 297 707.A3 entitled "Superconductive electric wire and method for making it"; European Patent Application Publication No. 0 286 372.A3 entitled "Oxide superconductor and manufacturing method thereo~"; and U.S. Patent No.
4,965,245 entitled "Method of producing a supexconducting wire including an oxide superconductor".
.
Recently, various superconductive oxides with high ~; critical temperatures (Tc) have been discovered. For pro-, 30 ducing superconducting wires including such superconductive ~' ~`
,~ , `'`' ~' .

'I 3 1 3 ~ ~ ~

oxides, for example, Y-Ba-Cu oxide, there has been proposed that a powder mixture which includes Y203 powder, BaO powder and CuO powder is charged into a metallic pipe, which is then diameter reduced to form a composite wire, which i9 in turn heat treated for a solid-state reaction so that the superconductive oxide is produced in the core. According to such a method, it is h~owever difficult to improve critical current density of the superconducting wire in spite of various efforts.

With intensive study, we have found that this difficulty is caused by a fact that during the heating treatment, the oxide core is damaged by tensile stresses due to the difference in thermal expansion between the metallic sheath and the oxide core, so that the superconductivity of the superconducting wire is degraded. Our experiments revealed that the sheathed superconducting wire above described exhibited 1/2 to 1/5 in critical current density of a bulk material of the same superconductive oxide.

Accordingly it is an object of the present invention to provide a method of producing a superconductor which exhibits exce~llent superconductivity, particularly high critical current density as compared to the superconductor produced by the prior art method.

Summary of the Invention :
With this and other objects in view, the present invention provides a method of producing à superconductor without a metallic sheath, the superconductor including a superconductive oxide. The superconductive oxide is represented by the formula AXByczD7-~;

,~ . .

!`
13~L3~3 provided that the A is at least one selected from the group - consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, the B is at least one selected from the group consisting of Be, Sr, Mg, Ca, Ba and Ra, the C includes Cu, the D includes 0, about 0.1 < x < about 2.0, about 1 < y <
about 3, about 1 < z < about 3, 0 < ~ < 5, or by the formula ~ xByCazCu i j provided that the A includes Bi or Tl, the B includes Sr or Ba, about 1 S x < about 3, about 1 < y ~ about 3, about O < z < about 3 and about O S i < about 4. A filling material is charged into a metallic pipe for forming a preform. The filling material is at least one material selected from the group consisting of a starting material powder of the superconductor, a powder of the superconductor and a compact made of the starting material powder and/or the superconductive pow~er. The preform is reduced in cross-sectional area for forming a composite including a core, made of the at least one filling material, and a metallic sheath covering the core. The metallic sheath of the composite is removed for exposing the core. The exposed core is heat treated for producing the superconductive oxide. The superconductor of the present invention may be in the form of a wire, multifilamentary wire, cable, ribbon, bulk shape and other like configurations.

Another aspect of the present invention is directed to a superconductor produced by the method above mentioned.

Brief Description of the Drawings - In the drawings:

FIG. 1 is an larged cross-sectional view of a preform according to the present invention;
.
~ .
.

~3 L3~32 FIG. 2 is an enlarged cross-sectional view of a modified form of the preform in FIG. 1;

FIG. 3 is an enlarged cross-sectional view of a composite wire produced by diameter reducing the preform in FIG. 1;

FIG. 4 is a diag~ammatic illustration of a rotary swaging machine, in which a composite wire of the preform in FIG.2 is introduced;

FIG. 5 is a diagrammatic illustration of another swaging machine for further swaying the composite wire processed in the swaging machine in FIG. 4i FIG. 6 is an enlarged cross-sectional view of the core obtained by removing the sheath of the composite in FIG. 3;

FIG. 7 is a perspective view of the induction heating appliance used in one preferable mode of the present invention;

FIG. 8 is a diagrammatic illustration of a modified form of the induction heating appliance in FIG. 7;

FIG. 9 is an enlarged receptacle for recovering a molten : metal in~the heating appliance in FIG. 8;
:
FIG. 10 is an enlarged cross-sectional view of a superconductor coated according to the present invention;

FIG. 11 is an.illustration of a hot dipping process used in one preferred mode of the present invention;

:

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1313~32 FIG. 12 is an enlarged cross-sectional view of a heat treated core coated with a buffer layer according to the present invention;

FIG. 13 is a cross-sectional view of the coated core in FIG. 12 sheathed with a metallic sheath;

FIG. 14 is a perspective view of an apparatus for producing a multifilamentary superconductor according to the present invention;

FIG. 15 is an enlarged cross-sectional view of the multifilamentary superconductor produced by the apparatus in FIG. 19; and FIG. 16 is an enlarged cross-sectional view of a modified multifilamer-tary superconductor in FIG. 15.
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~3~3~32 Sa Detailed Description of the Invention ~he s~percond~ v~ o~ide In the AxB~CzD7_~ superconductor of the present invention, the C may include Cu or Cu plus at least one element among Ag, Au and Nb and the D may contain O or O plus at least one element ~ong S, Se, Te, Po, F, Cl, Br, I and At.
Specific examples are: Bao.lsro.o5Lal.5ybo.35cuo3.2Fo~8;
Ba0.1Sr0~0sLal.sYbo.35cuo.9Ago.lo3.2Fo.8 ; and Ba0.1Sr0.05Lal.5Ybo.35cuo. gAuo .103 2Fo.g. In YxBayCuz07_~
superconductor, preferably x=l, y=2, z=3, 0 < ~ < 1, typically is about 0, and the oxide superconductor is orthorhombic.
In La2_kBkCuOg, preferably 0 < k < 0.3 and typically, k =
0.15. Typical examples of the AxByCazCuiOj are Bi2Sr2Ca2CU3j, Bi2Sr2CalCu20j, TllCa2Ba3CuqOj, T12Ca2Ba2Cu30j, T12Ca2Ba1Cu30j. Other typical examples of the superconductive oxide according to the present invention are La2Cu104_m, BaKBiO3, and BaPbBiO3.

The ~illina m~t~rial The filling material according to the present invention may include: a starting material power, including the elements which constitute the oxide superconductor; a green compact of ~uch a starting material power; calcined green : com~act of the starting material powder; and a superconducting material obtained by sintering the green compact or by pulveri~ing the sintered compact. The filler may be in ~he orm of powder, granule, compacted body of such a material or a mixture thereof.

The startlng material power may contain: for example, a mixture;of a powder of the A element or elements, a power or a carbonate of the B element or elements, and a powder of the C element or elements; a pulverized, calcined powder of such a . ~

mixture; or a like powder. The powder of IIIa group elements, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, may be in the foxm of a powder of a compound such as a carbonate, oxide, chloride,. sulfide, oxalate and fluoride thereof and in the form of an alloy powder thereof. As the IIIa group powder, an oxide powder thereof with a particle size about 5 ~m or smaYler is preferahly used. The powder including Bi may be a powder of an oxalate thereof and the powder containing T1 may be a powder of T1203. The powder of IIa group elements, may be in the form of a powder of a compound such as a carbonate, oxide, chloride, sulfide, oxalate and fluoride thereof and in the form of an alloy powder thereof. As the IIa group powder, a carbonate powder thereof with a particle size about 3 ~m or smaller is preferably used. The powder containing copper may be a powder of a copper oxide including CuO, Cu20, Cu203 and Cu403. A CuO
powder of a particle size of about 3 ~m or smaller is preferably used. The ~lixing ratio of these compounds depends on a desired superconductor. For YlBa2Cu307_~, Y203, BaC03 and CuO powders are preferably mixed so that Y : Ba : Cu = 1 :

2 : 3 at mole ratio. The starting material powder may have a particle size of about 4 ~m or smaller, preferably about 1 to about 2 ~m. Within such a preferable rangej excellent heat diffusion of elements of the superconductive oxide may occur.

The starting material powder for the A-B-Cu-0 superconductor may be prepared by the following so-called sol~
gel method~ An aqueous solution of the A element f the B
element and Cu is prepared by weighing a soluble salt, such as a nitrate and acetate, of these elements at predetermined ratio, and then by dissolving them into a predetermined amount of water. The total concentration of the salts of these elements in the aqueous solution is preferably about 0.5 to about 10 wt.% but it depends on the kind of the soluble salt.
Such an aqueous solution may be prepared by dissolving an ~3~3~

oxide or carbonate of each element by an aqueous solution of nitric acid or acetic acid. Then, an acid, preferably carboxylic a~id such as citric acid, succinic acid and tartaric acid, is added to~the aqueous solution of the elements. About S to about ~0 wt.~ of citric acid is used per 100 wt.% of the aqueous solution. The amount of the other acids depends on the k'lnd thereof. The acid added aqueous solution is then neutralized by adding a basic material, such as ammonia, ammonium carbonate, guanidine and ammonium acetate, to obtain a neutralized aqueous solution of about pH
7. As the basic material, an aqueous ammonia is preferably used. Then, the neutralized aqueous solution is heated to evaporate water and further to decompose or pyrolize the acid material and basic material, so that a solid sponge material (mixture) of oxides or carbonates, such as Y203, BaC03 and CuO, of each element of the oxide superconductor is obtained.
Subsequently, the sponge material is heated for burning and is . .. . .. .
then pulverized by a ball mill or automatic mortar for a predetermined particle size. The burnt sponge material is an aggregate of fine particles with a particle size of about 0.1 to about 0.6 ~m and hence it is easy to produce fine powder mixture of a particle size of about 0.1 to about 0.6 ~m by pulverizing. The fine powder is calcined as described hereinafter.

An alternative method for preparing the starting material is the following so-called coprecipitation method, in which an aqueous solution of the elements is prepared in the same manner as the sol-gel method above-described. A precipitant, such as oxalic acid, potassium oxalate, potassium carbonate and sodium carbonate, is added to the aqueous solution. The amount of the precipitant depends on its kind. Precipit~tion is carried out by controlling pH of the aqueous solution by adding a basic material, such as an aqueous a~monia, ammonium carbonate and potassium hydroxide. When oxalic acid is used 3 ~

as the precipitant, pH is set ~o about 4.6 and when potassium carbonate is used, it is controlled to about 7 to about 8.
The coprecipitate is heated at about lO0 to about 200C, preferably about 150C, for drying and then it is calcined about 700 to about 900C for about 50 hours in a flowing oxygen atmosphere preferably including about 90 vol. % or more of oxygen. Then, the ~calcined material is pulverized by a ball mill or a mortar for a predetermined particle size of the starting material powder..

The filling material may be calcined at about 500 to about 1000C for about 1 to about 100 hours in an oxygen atmosphere, including oxygen gas wit~l an oxygen purity of 90 %
or more, preferably nearly 100 ~, ~or removing carbonates and carbon which are contained in it. When the high purity oxygen gas is forced to flow within the calcining furnace without standing, no significant problems are encountered but preferably, ~he flo~ rate is about qO cm/min or more. The calcining may be repeated if necessary. Subsequently, the calcined filling material may be pulverized for a predetermined particle size, for instance, with a ball mill, mixed and then pressed .into a bar-shaped compact by conventional methods, for example, cold hydrostatic pressing, such as rubber pressing using a rubber shell and hot hydrostatic pressing, for providing a predetermined green density. The compacting pressure may be about 1.5 to about 10 metric tonstcm2, pxeferably about 1 to about 5 metric tons/cm2 although it depends on the kind of the calcined material and on the predetermined green density. The calcining, the pulverizing and pressing operations may be repeated. With such operations, a green density of the compact may b~ 60 %
or more o~ the theoretical density which has zero porosity.
It is preferable to obtain a compact of a green density about 70 % or more of the theoretical density.

:~3~3~32 g The calcined, pulverized filling material may be charged into a rubber tube, having one closed end, which is evacuated in a va~uum ~hamber to a vacuum level, for instance, about 10-4 mmHg, for reducing blowholes in the core heat treated and then is sealed by closing the other open end also in the vacuum chamber. The sealed tube may be wrapped with a soft synthetic resin sheet 'such as of a polyvinyl chloride resin for enhancing sealing thereof. Then, the rubber tube, wrapped with the synthetic resin, is pressed by means of a hydrostatic rubber press machine to form a compact in the same manner as in the forming of the bar-shape compact above described. The compact thus prepared has little air holes and hence has a relatively high green density and little cracks. This compact may be subject to the subsequent intermediate sintering described below. Then, the compact may be heated at about 700 to about 1100C for about 1 to about 100 hours, preferably at about 800 to about 1300C and more preferably at about 850 to about 950 C for about 1 to about 50 hours, in an oxygen atmosphere. With such an intermediate sintering, the sintered compact may have a sintered density of about 75 % or more of the theoretical density. This sintered density of about 75 %
or more provides preferable sintered density, that is, about 82 % or more, of the sintered core of the composite wire by a heat treatment with ease after subsequent forging or cross-sectional area reduction which will be hereinafter described.

When the sin~ered density of a sintered compact which has been subjected to the intermediate sintering is set to about 70% to about 75%r the diamet~r reduced core thereof may have a green density of about 75~ to 85% of the theoretical density which provides a sufficient amount of oxygen to the inside of the core 22 having no sheath during subsequent heat treatment for producing a superconductive oxide, so that the sintered core having an excellent superconductivity may be produced ~, 3 ~

with a sintered density of about 90% or more of the theoretical density.
.

The filling material of the superconductive material may be prepared by calcining the starting material powder at about S00 to about 1000C f~r about 1 to about 50 hours, pressing the calcined powder to form a compact in a similar manner above described, and then heating the compact for about 700 to about 1100C for about 1 to about 100 hours in an oxygen atmosphere or oxygen-containing atmosphere, which will be stated in more detail in paragraphs entitled "The heat treatment", for producing a superconductive oxide. For Y-sa-Cu oxide su~erconductor, the co~i~act is heated prefe~ably for about 800 to about 1000C for 1 to about 50 hours.
Thereafter, the heat treated compact is pulverized to obtain a predetermined particle size of the superconducting material powder. These pressing, heating and pulverizing operations may be repeated for producing a superconducting material powder of a homogeneous composition. The superconducting material powder is selected with a conventional method, such as sedimentation, to have a particle size of, typically, about 1 ~m or smaller and preferably about 0.7 ~m to about 1.5 ~m.
The superconducting material powder thus selected may be pressed and then subjected to intermediate sintering in the same manner as previously described.

met~,lLi~ ~ipe The metallic pLpe according to the present invention may be made of, for example, copper, a copper alloy, a noble metal such as silver, gold and platinum, an alloy of such a noble metal,~ aluminum and a stainless steel. The pipe may be made of other metals or plastic materials other than metals.

~ .

~3~ 3~3~

The thickness of the metallic pipe is preferably about 10 to about 25 % of the outer diameter thereof. The lower limit of the thickness of the metallic pipe should be such that it does not produce a breaking of the diameter reduced composite wire having a predetermined diameter. The upper limit is determined in view of/both the pressure transmittance to th~
core and the cost of the metallic pipe.

Thç pr~orm The filling material is charged into the metallic pipe to form a preform. FIG. 1 illustrates a preform 3 which may be, according to th~ present invention, prepared by insertin~ a compact 2, made of a superconductor powder, into the metallic pipe 1. The cornpact 2 may be made by presseing and then sintering a green compact of the superconducting material into a cylindrical-shape. The temperature of the sintering may be about 400 to about 1000 C- The compact 2 may be made with a rubber shell in a conventional hydrostatic press machine. It is preferable that the gap between the compact 2 and the metallic pipe 1, which fits around the compact 2, is as small as possible so that forging pressure may be sufficiently applied to the core compact in the subsequent diameter reduction.

As illustrated in FIG. 2, the filling material 2 may be, according to the present invention, charged into a metallic pipe 1 into which a core wire 4 is concentricallv arranged to form a preform 5. The core wire 4 is made of a non-oxidizing material which does not take oxygen away from the filling material 2 in the metallic pipe 1 during the subsequent heat ~reatment. The core wire 4 should have a high tensile strength with a melting point higher than about 800C and may include, for example, a metal wire, such as of silver, gold, platinum, titanium, tantalum and a silver alloy, and a ceramic ~ 3~3~

fiber such as a carbon fiber, silica fiber and alumina fiber.
The cross-sectional area of the core wire 4 has preferably about 10 % or less of the cross-sectional area of the filling material 2 charged in the metallic pipe 1. With about 10 % or less, the core wire 4 provides excellent effects to the superconductor in rai~ing green density of the core of the composite wire and in mechanical strength thereof.

The diameter reduction In the present invention, the preforms 3 and 5 may be conventionally diameter reduced and formed into a composite wire 6 by well-knowll metho~s, for example, drawlng with a di~, rolling with grooved rolls or swaging such as rotary swaging, to a predetermined diameter. The diameter reduced composite wire 6 has a ~etallic sheath 7 and a core 8 sheathed with the sheath 7. The diameter reduction operation may be repeated.
It is preferable that the forging ratio F is within a range about 10 % to about 40 % ~or each diameter reduction operation in which F is defined by the formula F = (S1-S2) x 100/S1 where S1 and S2 are cross-sectional area of the preform 3, 4 and the diameter-reduced preform or composite wire 6, respectively. Below about 10 % of the forging ratio F, the number of the diameter reduction operation is rather increased. Beyond about 40 %, it takes rather long period of the processing time.

The preforms 3 and 4 are preferably diameter reduced by rotary swaging using a conventional rotary swaging machine A
as in FIG. 1, in which a plurality of dies 10 are arranged about an axis X thereof and are forced to be axially moved (in the direction of the arrow ~) during rotating about the axis X
(in the direction of the arrow b). The rotary swaging machine is arranged so that the dies 10 surround the traveling path .

~3~ 3~3.~

of the preform 5. The dies 10 are supported to be movable perpendicularly to and to be rotatable about the traveling path. Each of the dies 10 has an inclined face 12 inclined to the axis X so that the inclined faces 12 thereof define a substantially conical working space 14 tapering forwards.
~' In diameter reduction, the rotary swaging machine A is actuated and then one end of the preform 5 is pushed into the tapering working space 14 of the rotary swaging machine A
along the traveling path thereof. The preform 5 is diameter reduced from its one end by the dies 10, which are radially reciprocated and rotated about the axis X, and it is thereby sha~ed into a composite wire 16 and hence th~ rotary sw~ging provides a fairly large ~orging ratio to the preform 5 as compared to other conventional forging methods. In this rotary swaging machine A, the processing speed or the -t~a~eling-speed of the preform 3, 5 through it is preferably about 0.1 m to about 10 m/min.

When needed, the composites 6, 16 may be further diameter reduced to a predetermined diameter by means of another rotary swaging machine B in FIG. 5 which has a conical working space 20 smaller than that of the first rotary swaging machine A. In this second diameter reduction, th~ composite 6, 16 is swaged from the other end to the one end while in the first diameter reduction, it has been diameter reduced from the one end to the other end. Such change in the swaging direction along the axis X provides an increase in green density o~ the core 8 in the sheath. The swaging operation ; may be repeated more than twice, in which case the direction of swaging may be changed in each operation or with intervals of a predetermined number of swaging.

The composite wire 6, 16 undergoes the rotary swaging until the green density of the core 8 reaches eo about 75 % or .~

~3~3~

1~
more, preferably to about 77 % or more of the theoretical density. Wlth the green density of less than about 75%, superconductivity of the produced oxide superconductor may be degraded since there is an upper limit of increase in density of the core in the the subsequent heat treatment, which will be described hereinaf~er. The core o~ composite wire 6, 16 may have a green density of about 75% or more by other conventional methods such as die forging.

~he removal ~f~ ~he metalliç sheath The metallic sheath is removed form the composite, diametel reduced, to expose Lhe core 22 thereof to the atmosphere as illustrated in FIG. 6.

The removal of the metallic sheath may be, according to the present invention, carried out by dipping the diameter - -reduced composite into a solution of an acid or an alkaline, as a treating liquid, for dissolving the sheath. More specifically, a strong acid such as dilute nitric acid may be used for dissolving a metallic sheath made of silver, copper or their alloy.

When an alkali soluble metal which is ssluble in an alkali solution is used for the metallic sheath, an aqueous solution of an alkali, such as sodium~hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate and potassium carbonate, may be adopted as the treating liquid. When aluminum is used for the sheath an aqueous solution of an alkali, such as sodium hydroxide, may be used. Conditions of removing of the metallic sheath depend on the material thereof. When aluminum or its alloy is used for the sheath, removal thereof may be carried out-at room temperatures. When iron or its alloy is used, the metallic sheath i5 heated, during which it is treated with a concentrated aqueaus - ' .

~ 3~32 solution of an alkali, such as sodium hydroxide in the existence of oxygen. Aqua regia may be used for dissolving a metallic sheath of a stainless steel. Hydrochloric acid may be used as the treating liquid according to the material of the metallic sheath.

For preventing impurities from entering into the superconductor and the manufacturing facility from being corroded from the treating liquid, it is preferable to wash the exposed core 22 with water or to neutralize the treating liquid, adhered to the exposed core 22, after washed with water.

In the present invention, the metallic sheath may be machined for removing it, but care should be taken not to break or damage the brittle core 22 particularly when the wire ... ... .... .. . . . .
6 lS fine.

Alternatively, the metallic sheath may be, according to the present invention, removed by high frequency induction heating to expose the core 22 and continuously the exposed core 22 may be heat treated for producing the oxide superconductor. In this removing method, the diameter reduced composite 6, 16 may be continuously introduced, as illustrated in FIG. 7, into a heating tube 30 having a glass tube 32, made of a heat-resisting glass, silica glass or a like glass, with an inner diameter of about 10 to ~0 mm and length of about 40 m. The glass tube 32 is arranged in an inclined manner such that the inlet of thereof is lower than the outlet thereof as shown in FIG. 7 so that a molten metal may flow out of the inlet. The heating tube 30 has high frequency induction heating coils 34, wound around the glass tube 32, and is provided with a plurality of, three in this embodiment, oxygen supply tubes 36 mounted to the glass tube 32 to communicate to the inside thereof. Thus, there are ~3:~ 3~3~

provided five heating zones, that is, a first heating zone 38a, second heating zone 38b, third heating zone 38c, fourth heating zone 38d and fifth heating zone 38e from the inlet to the outlet. High frequency current of about 5 kHz to about 500 kHz is supplied from a power source to respective coils 34 to provide outputs of~bout 1 kW to about lO0 kW. In this heating tube, high frequency current of 25 kHz is supplied to the first, second, third, fourth and fifth heating ~ones 38a-38e to yield outputs of 30, 10, 5, 1 and 1 kW, respectively.
The length of the first heating zone 38a is about 10 m and the length of each of the other heating zones 38b-38e is about 5 m. When the composite 6, 16 is introduced into the energized first heating zone 38a of tn~ heating tube 30, eddy current is generated in the metallic sheath 7, so that the latter is melted and removed from the composite 6, 16 to thereby expose the core 8. In this event, no substantial eddy current is generated in the core 8 since it has-a volume resistivity of 10-3 to 1 Q-c~ and hence it gradually heated by dielectric loss. Then the core 8 is subsequently moved to the second to fifth heating zones 38b to 38e. Since outputs of the heating zones 38a to 38e are gradually reduced, the core 8 is heated at the highest temperature, in this heating tube 30, of about 900C and then gradually cooled. The speed of the slow cooling depends on the output and the len~th of each heating 20ne 38a-38e and traveling speed of the core 8 in the heating tube 30. For preventing cracks from occurring due to rapid cooling, it' i9 preferable to gradually cool the core 22 at a speed of about -50 to about -500C/hour while it is cooled from about 900C to about 400C. This induction heating is carried out in an oxygen atmosphere. More specifically, hot oxygen gas which is previously heated is introduced into the glass ~ube 32 ~hrough the oxygen supply tubes 36 to form the oxygen atmosphere, in which the exposed core 22, from which the metallic sheath 7 has been removed, is induction heated and then gradually cooled by the high frequency induction J ~
~, , ~3~3~

heating coils so that an oxide superconductor with fine crystal structure is produced. In cas~ of production of Y-Ba-Cu-O superconductor, the transformation from a cubic system to a rhombic system thereof may be smoothly carried out with this heating tube 30. Then, the exposed core 22 is pulled out of the heating tube 30 a~d is preferably cooled at a speed of about -50 to about -500 C/hour for preventing cracks from being produced due to rapid cooling. The slow cooling may be car~ied out in a furnace using a conventional heater without use of the second to fifth heating coils 38b-38e. The core 22 which is issued out of the heating tube 30 may be further heat treated for annealin~.

The molten metal of the m~tallic sheath 7 may be transported to the outside of the heating tube 30 by arranging the latter in an inclined man~er so as to discharge it by gravity. Alternatively, a molten metal receiving tape may be introduced into the heat tube 30 so that it passes just below the composite 6 for receiving the molten metal of the metallic pipe 7 and then, the tape is pulled outside to issue out from the heat tube 30 for recovering the molten metal.

A modified form of the heating tube 30 in FIG. 7 is illùstrated in FIG. 8 in which another heating tube 40 is vertically arranged. The composite wire 6 is pulled out from a reel 42 and is then introduced via a roller 94 into the vertical heating tube 40. The wire 6 or the core 22 concentrically passes through the heating tube 40, where the metallic sheath 7 is melted at heating zones and then the exposed~core 22 is gradually cooled at the slow cooling portion 42b at an appropriate speed of about ~50 to about -500 C~hour. The molten metal falls to the bottom of the heating tube 40 where it is preferably collected in a cup 44 (FIG. 93 coaxially located just below the heating tube 40 although the cup 44 is not illustrated in FIG. 8. The composite wire 6 is 3~

l8 pulled upwards and coaxially passes the cup 49 through a hole 46 formed through the bottom thereof. The cup 44 has a discharge pipe 48 jointed to its bottom 44a for discharging the collected molten metal to the outside. The exposed core 22 which has issued from the heating tube 40 is subjected to dip forming at a bath~E for ~orming a coating 50 (FIG. 10) and then is wound via a roller 52 around a winding reel 54. In this heating tube 40, the wire 6 and the core 22 vertically passes through the heating tube 40 and hence it is kept vertically without excessive tension for pulling it. This is more preferable for preventlng cracks due to tension from occurring than in the heating tube 30 in FIG. 7 where the wire 6 and the core 22 should be kept ti~ht not to touch the inner face of the heating tube 30.

The heat treatment - .. ~ .. . .. . . .
Aft~r the metallic sheath 7 is removed, the exposed core 22 may, according to the present invention, undergo the heat treatment outside the heating tubes 30, 40 to produce a superconductive oxide without being subjected to the heat treatment within the heating tubes. The heat treatment may be made in an oxygen atmosphere with oxygen content of about 90 volume % or more at about 800 to about 1100C for about 1 to about 500 hours. This heat treatment is preferably carried out at about 850 to about 920C for about 1 to about 100 hours. Below about 850C, it takes a considerable time to increase the sintered density and above about 920C, the ~crystal grain of the oxide sup~rconductor is liable to have a columnar structure and hence clearances between crystal grains may become relatively large, so that the sintered density may decrease. For producing a Y-Ba~Cu oxide superconductor, after the heat treatment the core is pre~erably gradually cooled at 100C/hour and may be maintained at about 400 to about 600C
for about 5 to about 50 hours for transforming a cubic system 3 ~ ~ ~c~
l 9 to a rhombic system of the crystal structure during the slow cooling. With the oxygen concentration of about 90 volume %
or more, excellent superconductor may be produced. The purity of the oxygen gas is preferably about 90 % or more and flow rate of such high purity oxygen gas may be about 0.5 to about 5 liters/min. The he~t treatment may be carried out in a pressurized atmosphere, in which the pressure of oxygen gas is preferably about 1.5 to about 5 atmosphere. The core 8 becomes an excellent oxide superconductor since it is exposed to the oxygen atmosphere and is supplied with a sufficient amount of oxygen from the atmosphere. Furthermore, the metallic sheath is removed from the core 8 during the heat treatment, arld hence any crack due to s~resses which may be caused by the difference-in thermal expansion coefficient between them occurs. When the preform is subjected to the rotary swaging as previously described so that the green density of the core 8 reaches to about 75% or more of the theoretical desity, the sintered density of the heat treated core 22 may become about 90 to about 95% of the theoretical density, which provides an excellent superconductivity to the finished superconductor. When the green density of the core of the composite is 82% or more, then the sintered density of the heat treated core may be about 91 % or more.

Instead of the oxygen atmospherel other gases, such as an oxygen gas mixture including, VIb group gas, such as S, Se, Te or Po gas other than oxygen gas, VIIb group gas, such as F, Cl or Br,~ or an inert gas, such as He, Ne, Ar, Kr, Xe or Rn ~gas, may be used for the;~heat treatment. With these gas mixture~atmospheres, such~elements may diffuse into the core and hence the heat treated core has at its surface portion a superconduc~ive oxide having a uniform superconductivity along ~` its axis. Thus, excellent oxide superconductor may be produced.

~3~ 3~3~

~he coatin.a tre.a~m~

After the heat treatment, the core 22 may be subjected to a coating treatment during application of ultrasonic waves and thereby a superconducting wire 52 having the core 22 coated with the coating laye~ 50 is obtained as illustrated in FIG.
10. The coating layer 50 may have a thickness about S~m to about lOO~m, and preferably about 10~ to about 50~m. The coating treatment may be made by electroplating, hot dipping and similar coating of: a solder, such as of an alloy of zinc and copper and an alloy of tin and lead; a low melting point metal, such as aluminum, tin, zinc, lead, indium, gallium and bismu~h, an alloy thereof; and a synth~tic resin such as a polyimideamide resin, formal, Teflon resin, nylon and polyvinyl chloride. Me~als, such as aluminum, having low electric resistance at liquid nitrogen temperature are preferably used for the coating metal. The metallic coating layer of such metals may be used as a stabilizing layer of the superconductor. Another specific coating technique is that a powder of such low melting point metals or their alloys is adhered to the surface of the heat treated core 22 to form a coating, which is then sintered. With the coating 50, elements, such as oxygen, of the superconductive oxide are prevented from leaving it and are protected from foreign meterials such moisture. Thus, the coating 50 maintains excellent superconductivity for a fairly long period of time.
The meltin~ temperature of the solders and low melting metals are below about 800 C, preferably belo~ the temperature at which the crystaI structure of the superconductive oxide in the core may be affected.
.

FIG. 11 illustrates a hot dipping process, in which the core 22, heat treated, may be continuously passed through a molten solder 60, such as of an alloy of zinc.and copper or an alloy of tin and lead, in a treating bath 62, and after a , .

13~3~32 predetermined period of time, it is taken up from the bath 62 and cooled to solidify the solder 60 adhered to the core 22, so that a superconducting wire 52 having a predetermined thickness of coating layer'is produced. An ultrasonic wave generator 64 may be mounted to the bath 62 for applying ultrasonic waves throygh the molten solder 60 to the core 22 passing through it. By applying ultrasonic wa~es, air or other substances adhered to the core 22 is removed from it for improving wettability thereof, so that the solder 60 is strongly bonded to the core. Ultrasonic waves h~ving a frequency of about 5 kHz to about 200 kHz are preferably used.

The coated oxide superconducting wire ~2 may further undergo a plating treatment for coating the coating layer with a metallic layer 70, made of tin, copper or a like metal, to reinforce the core 22 as in FIG. 10.

As illustrated in FIG. 12, the heat treated core 22 may be coated with a buffer layer 72 for reducing thermal stresses which are produced in it when it is cooled at liquid nitrogen temperature. In this case, the coating layer and metallic layer are omitted. The buffer layer 72 may be made of a substance which is intermediate in coefficient of thermal expansion between the heat treated core 22 and the a metallic ~sheath 74 which will be described later. The metals or alloys above ment~oned may be used for the buffer layer 7~. The buffer~layer 72 may be formed by winding a tape, made of such materials, around the heat treated core 22 or placing it to surround the core so that the tape extends along the axis thereof. Hot-dipping, vapor deposition and dip forming may be also adopted for forming the buffer layer 72. The metallic sheath 74, such as of aluminum and copper, may be formed around the buffer layer 72 to form a sheathed superconductor wire 76~ The metallic sheath 74 may be formed by covering the buffer layer 72 with a tube, made of a tape or a thin plate, , .

~, : . . , ,~. .. . .. .

~3~3~

by means of a conventional sheath forming method using dies or forming rolls without forming a gap between the metallic sheath 74 and the buffer layer 72. The superconducting wire 76 thus fabricated may be wound around a core of a superconducting magnet as a coil or may be used for power transmission. J

Prod~c~ion ~ multifilament~ry superconductors A plurality of, several tens, coated superconductors 52 are arranged through respective holes 121b which are formed through a first separator 121 with regular intervals as shown in FIG. 14. The first separator 121 ma~es the coated superconductors 52 straight and places them in parallel to each other. Then, the superconductors 52 pass through holes 122b formed through a second separator 122 with predetermined regular intervals for arranging them in a bundle 110 with predetermined regular intervals, which this then passes through a molten metal bath 123, which contains a molten metal M which is the same kind of metal as the coating metal 50 hereinbefore me.ntioned. The superconductors 52 enter the bath 123 sealingly through bushings of the inlet 129, which keeps the bundle in the predetermined regular intervals, and issue from it through dies of the outlet, the dies sealing the molten metal M. Roller dies are preferably used as the dies of the outlet for reducing load which is applied to the superconductors 13~ when they come out of the bath 123. The bath 123 has a ultrasonic wave generator 126 at the inner bottom thereof for vibrating the molten metal M and heaters 127 below the bottom for heating the molten metal M. When the bundle 110 of the superconductoxs 52 issues from the bath 123, the molten metal M, a~hered to it, is solidi~ied to form a bundle coating 125 coa~ing it, so that a multifilamentary superconducting wire 130, as illustrated in FIG. 15, is fabricated. ~en the superconductors 52 have a large .
.,~ .

~3~32 mechanical strength, the bundle 110 thereof may be twisted for enhancing mag~etic stability of the multifilamentary superconductor 130. In such a case, the first and second separators 121 and 122 may~be synchronously rotated at low speeds for twisting the bundle 110 between the second separator 122 and the~putlet of the bath. The molten metal may be metals previously mentioned in connection with the coating metals 50. The multifilamentary superconductor 130 is water cooled with a water cooling device 128. when the bundle coating 125 is made of aluminum, it serves as a stabiliæer when superconductive state of the superconductor is broken.

When the coating 50 and the bun~le coating 125 are nlade of different metals, a multifilamentary superconductor 132 as illustrated in FIG. 16 is fabricated. In this case~ the coating 50 may be ~ormed of a metal having a melting point higher than that of the bundle coating 125.

Instead of the coated superconductors 52, uncoated superconductors 22 may be used for producing a multifilamentary superconductor in a similar manner.

Oth~_preferred mQ~es of th~ InventiQn It has been discovered that when superconducting materials are calcined or sintered at high temperatures for a long period of time for enlarging crystal grains, clearances between crystal grain may excessively increase, so that oxide superconductors produced may be rather degraded in critical current density. For reducing this drawback, calcining, intermediate sintering and sintering operations may be carried out in the following conditions although other production conditions of which description is omitted are the same as as hereinbefore described. The filling material may be calcined about 800 to about 950~C for about 6 to 50 hours. Under these ~31 3~32 conditions, the particle size of the calcined material may be 10 ~m or smaller. The calcining temperature is preferably about 850 to about about 920C. Within this temperature range, the calcined material may have a particle size of 5 ~m or smaller, which material facilitates the producing of a superconductlng materlal having a particle size of 10 ~m or smaller after intermediate sintering. In the intermediate sintering, a compact, which may be prepared in the same conditions as previously described in the subtitle "The filling material", is subjected to the intermediate sintering at about 800 to about 950C, preferably 850 to about 920C, for about 6 to about 50 hours in oxygen atmosphere and then may be gr~dually cooled to the-eby pro~uce an intermediate sinter rod. The preferable temperature range facilitates to produce an intermediate sintering with fine crystal grains equal to 10 ~m or smaller. After the sheathing, cross-sectional area reduction and removing of the sheath, the exposed core undergoes the heat treatment at about 800 to about 950C, preferably 800 to about 920C, for about 6 to about 50 hours in oxygen atmosphere for producing an oxide superconductor with fine crystal structure.

A solidified material which is made of a molten starting material may contain excellent oxide superconductors and from the solidified material an excellent superconductor with relatively high critical current density may be produced.
According to this modified method, the superconductor is produced in the same conditions as previously stated except the following. The starting material powder, already stated in connection with "The filling matèrial", is calcined at about 750 to about 950C for about 3 to about 50 hours and then pulverized to a predetermined particle size.
Subse~uently, the calcined powder is subjected to intermediate sinterlng at about 800 to about 950C for about 3 to about 50 hours and then cooled to produce a superconductive oxide .
.
..

~3~3~3~

powder. For producing Bi-Sr-Ca-Cu oxide superconductor, the calcined powder undergoes intermediate sintering preferably at ~90C for 20 min. and then at 880 C for 9 hours, after which it is rapidly cooled. For Y-Ba-Cu oxide superconductor, the slow cooling previously described, which includes transformation to a r~ombic system, is preferably made. The sintered powder is placed in a platinum or CaO crucible where it is heated at about 1300C ln an oxygen containing atmosphere to produce a molten material, which is solidified by rapid cooling to a temparature of about 800 to about 950C.
This rapid cooling may be carried out by taking out the crucible, containing the molten material, from the heating appliance an~ placing it ln the ~tmosphere. Alternatively, the crucible may be forcedly cooled by using a coolant. After maintained at the temperature about 800 to about 950C for about several to about several tens hours, the solidified material is cooled to room temperature. The cooling to the room temperature may be rapidly made for Bi-Sr-Ca-Cu-O system superconductor. The slow cooling as previously described, which includes transformation to a rhombic system, is preferable for Y-Ba-Cu oxide superconductor. A surface portion of the solidified material thus obtained is cut with a thickness 1 mm or smaller, preferably about several ~m to about several hundreds ~m by machining and is then pulverized to produce a surface powder which contains hi~h purity and homogeneous superconductive oxide. The remaining solidified material is remelted and reused for obtaining the surface power in the same manner. Such a powder may be directly obtained by injecting the molten material into the atmosphere at about 800 to about 950C with a carrier gas. However, the powder should have a particle size of about several hundreds ~m or smaller. These powders, obtained from the solidified material, ma~ be pressed to form the bar-shaped compact, already stated, which may be heat treated at about 800 to about 950C for about 6 to about 50 in an oxygen atmosphere to , l ~3~L3~32 increase the content of the oxide superconductor. After subjected to sheathing, cross-sectional area reduction and removal of the sheath, the exposed core is sintered at about 800 to about 950C for about 6 to about 50 in an oxygen atmosphere.
"

~3~3~3~

Example 1 Powders of Y2O3, BaCO3 and CuO were mixed at mole ratio Y: Ba: Cu = 1 : 2 : 3 to obtain a starting material powder mixture, which was ca~cined at 900C for 24 hours in the atmosphere and then pulverized to produce a calcined powder.
This calcined powder was pressed by a rubber press to form a compac~, which was heated at 900C for 24 hours and then gradually cooled to produce a rod containing a superconductive oxide YlBa2Cu307_~ of which critical current density was about 40A/cm2. The rod had a sintered density of about 75%. The rod was inserted into a silver pipe having an out~r diameLer 10 mm and inner diameter 7mm to form a preform, which was cold forged in a stepwise manner by a rotary swaging machine as illustrated in FIGS. 4 and 5 to produce a composite wire with a diameter 1.5 mm without breaking. This cold forging was carried out at a processing speed of 1 m/min with a forging ratio of about 20~ for each diameter reduction. The sintered density of the core of the composite was more than about 75%.
The silver sheath of the composite wire was removed by placing it in nitric acid to expose the core. The exposed core was heated at 850 to 950C for 24 hours and then gradually cooled to room temperature at a speed of -100C/hour to produce a superconductor. The superconductor exhibited a critical temperature (Tc) of 91K and a critical current density (Jc) of about 10000 Atcm2 at 77K. This superconductor could be wound around a magnet core without crack and showed a sufficient mechanical strength.

Example lA

A superconductor was prepared in the same manner and conditions as in Example 1 with the starting material powders of a particle size about 2 ~m. In this Example, the ~3~3~32 pulverized, calcined powder had a particl size about 10~ and was pressed at 3 metric tons/cm2 to form the compact, having a green density about 65% of the theoretical density, which compact was heated in ambient flowing oxygen of 2 liters/min and subsequently gradually cooled at -200C/hour to produce the superconducting r~ which had a sintered density about 75~. The exposed core was subjected to the final heating in ambient flowing oxyqen of about 2 liters/min and then gradually cooled to produce an oxide superconductor, of which sintered density was about 95% of the theoretical density.
The superconductor thus produced exhibited a critical temperature of 91K and a critical current density of about 10000 A/cm2 at 7~K.

Comparative Tests 1 and 2 Two oxide superconductors were prepared in the same conditions and manner as the Example 1 except that sintered density of compacts for Comparative Tests 1 and 2 were 65% and 70% respectively. The superconductors of Comparative Test 1 and 2 exhibited critical current density of 200 A/cm2 and 500 A/cm2, respectively.

Example 2 Powders of Y2O3, BaCO3 and CuO were mixed at ratio Y: Ba:
Cu = 1 : 2 : 3 to obtain a powder mixture, which was calcined at 900C for 24 hours in the atmosphere. This calcined powder was pressed to form a compact, whish is heated at 900C ~or 24 hours and then pulverized. These pulverizing, pressing and heating operations were repeated three times to produce a superconducti~e powder cvntaining a superconductive oxide Y1Ba~Cu3O7~~, from which a superconducting powder of 0.5 to 1 ~m particle size was obtained by coprecipitation method and was pressed by hydrostatic pressing at pressure of 2.5 ton/cm2 . ~
'~-\

~3~32 to produce a rod-shaped compact of 6.5 mm diameter, which was heated at 900C for 24 hours in an oxygen atmosphere to obtain a first sinter, of which sinterlng density was 75% of the theoretical density. The ~irst sinter was inserted into a silver pipe having the same silver pipe in Example 1 to form a preform, which was co~ forged in the same manner as in Example to obtain a 1.5 mm diameter composite wire without breaking thereof. The sintered density of the core of the composite wire was about 80% of the theoretical density. The composite wire was subjected to the removing of the silver sheath, the heat treatment and the slow cooling in the same conditions as the Example 1 except that the final h~at treatment was carried out for 12 hours instead of 29 hours.
The heat treated, exposed core was coated with a 1 mm thick solder coating by plating to produce a superconductor. The superconductor was equal in superconductivity in critical temperature and critical current density to the superconductor of Example 1 and also showed an excellent mechanical strength in winding around a magnetic core.

Comparative Tests 3 and 4 Two oxide superconductors were prepared in the same conditions and manner as the Example 1 except that sintered density of the core in the sheath was smaller than 75% and sintered density of compacts for Comparàtive Tests 3 and 4 were 80% and 85% respectively. The superconductors of Comparative Tests 3 and 4 exhibited critical current density of 200 A/cm2 and 500 A/cm2, respectively.

Example 3 Powders of Y2O3 with particle size 4 ~m or smaller, BaCO3 with particle size 1 ~m or smaller, and CuO with particle size 1 ~m or smaller, of which each powder had purity of 99.9~ or ;~

~3~3~32 more, were mixed with a ball mill at mole ratio Y: Ba: Cu = 1 : 2 : 3 to obtain a powder mixture, which was calcined at 900C for 24 hours in ambient flowing oxygen. This calcined powder was pulverized and then pressed to form a round bar shaped compact at 2500 Kg/cm2. This series of heating, pulverizing and press~ng operations were repeated three times to produce a 6.5 mm diameter bar-shaped calcined compact, of which sintered density was about 90% of the theoretical density. The rod was inserted into the same silver pipe as in Example 1 and was obtained a 1.5 mm diameter in the same manner as in Example 1 except for forging ratio of 10% per each pass. The sintered density of the core of the composite wire was about 80~ of the theoretical density. The compos;te wire was subjected to the removal of the sheath, the final heating and slow cooling in the same conditions and manner as in Example 1 except that the final heat treatment was carried out at 890 C for 17 hours. The superconductor thus~produced, of which sintered density was 93% of the theoretical density, exhibited a critical temperature of 91K and about 11,000 A/cm2 at 77K and also showed an excellent mechanical strength in winding a magnetic core.

Example 4 A calcined compact of which green density was about 62 %
was prepared in the same conditions and manner as in Example 3, and then hea~ed at 900C fox 24 hours in ambient flowin~
oxygen with subsequent slow cooing to produce a round-rod-shaped intermediate sinter, containing a superconductive vxide Y1Ba2Cu3O7_~ (0 S ~ 5 5), of which sintered density was about 72%. The intermediate sinter was inserted into a silver pipe having the same silver pipe in Example 1 to form a preform, which was cold ~orged in the same manner as in Example to obtain a 1.5 mm diameter composite wire without breaking thereof. The compos1te wire was subjected to the removing of .

.

~31L3~3~

the silver sheath, the heat treatment and the slow cooling in the same conditions as the Example 1 except that the final heat treatment was carried out at 890C for 17 hours. With the final heat treatment, a sinter with a sintered density of about 92% was obtained. The superconductor thus produced exhibited a critical t~mperature of 91K and about 11,000 A/cm2 at 77K and also showed an excellent mechanical strength in winding a magnetic core.

Comparative Tests 5 and 6 Two oxide superconductors were prepared in the same conditions and manner as the Example 4 except 'hat green density of the calcined compact for Comparative Test 5 and 6 were 50% and 55% , respectively. The superconductors of Comparative Tests 5 and 6 had sintered density of 80% and 85 and exhibited critical current density of 200 A/cm2 and 500 A/cm2, respectively.

Example 5 The compact was prepared in the same conditions and manner as in Example 2 except that the powder mixture was calcined in a heating furnace with flowing 100 % purity oxygen of 80 cm/min. The pulverizing, pressing and heating operations were repeated also three times to obtain the compact, which was inserted in the same sil~er pipe as in Example 2 and then diameter reduced by using a rotary swaging machine to produce a composite wire with 1.5 mm outer diameter. The sheath of the composite wire was removed with an acid to expose the core, which was heated at 890C for 17 hours and then gradually cooled to produce a superconductor wire, of which critical curren~ density Jc at 77K and oxygen defect rate ~ are given in Table 1.

'~

~3~3~3~

Comparative Tests 7-10 Oxide superconductors were prepared in the same conditions and manner as in Example 5 except that calcining was carried out for oxygen concentration of 21%, which corresponds to the at~osphere, to 80~. The critical current density Jc at 77K and oxygen omission rate d of each superconductors are given in Table 1, from which it would be clear that a superconductor calclned with an oxygen concentration of 90 % or more provides excellent superconductivity.

Table 1 Comparative Tests Ex.5 7 ~ 9 10 Oxygen Concentration(%) 100 80 60 - ---40^-- 21 Jc (A/cm2) >104 2000 1200 920 640 d 0.05 0.19 0.31 0.35 0.40 Example 6 Powders of Y2O3, BaCO3 and CuO were mixed at ratio Y: Ba:
Cu = 1 : 2 : 3 to obtain a powder mixture, which was calcined at 900C for 24 hours in ambient flowing oxygen and then pulverized to produce a calcined powder. This calcined powder was~pulverized and-~then placed within a rubber tube, having 7 mm inner diameter, which was in turn pressed by a rubber press at 2.5 ton/cm2 to form a compact. The compact was heated at 900C for 24 hours. These pulverizing, pressing and heating operations were xepeated three times to produce a sinter with an outer diameter 6.9 mm, of which density was 4.5 g/cm3. The sinter was placed within the same silver pipe as in Example 1 and then subjected to cold forging, removal of the sheath and the final heat treatment also in the same conditions and ~ 3~3~32 manner as in Example 1 except that the core was heated at 900C for 12 hours in an oxygen atmosphere. The core thus obtained was coated by plating with a 1 mm thick solder protection layer. was carried out at a processing speed of 1 m/min with a forging ratio of about 20% for each diameter reduction. The sintere,d density of the core of the composite was more than about 75%. The silver sheath of the composite wire was removed by placing it in nitric acid to expose the core. The exposed core was heated at 850 to 950C for 24 hours and then gradually cooled to room temperature at a speed of -100C/hour to produce a superconductor. By repeating these operations, samples of the superconductor was prepared.
Densities of the cores after the sheath removal and heat treated are given in Table 2 as well as its critical current density Jc at 77K.
:
Comparative Test 11 Oxide superconductors were prepared in the same conditions and manner as in Example 6 except that the calcined powder was directly inserted into the silver pipe without being pulverized and pressed. The density of the core before swaging was 3.5 g/cm3. Densities of the core after the sheath re~ovaI and heat treated are given also in Table 2 as well as its critical current density Jc at 77K.

Table 2 Ex~ 6 Comparative Test 11 Density after removal of the sheath (g/cm3) 4.9 5.1 4.0~4.3 Density after the heat treatment~g/cm3) 5.8-5.9 5.2-5.4 Jc (A/cm2) 7000-10000 40-980 Example 7 Superconductors were prepared in the same conditions and manner as in Example 6 except that the diameter reduction was conducted by drawing with a die for each sample. The experimental results ~re given in Table 3.

Comparative Test 12 Superconductors were prepared in the same conditions and manner as in Comparative Test 11 except that the diameter reduction was conducted by d~awing with a die for each sample.
The experimental result~ ~re given in Table ~.

Table 3 Ex. 7 Comparative Test 12 Density after removal - -of the sheath (g/cm3) 4. 7-9 . 9 3.9 Density after the heat treatment(g/cm3) 5.6-5. 7 5.0-5.1 Jc (A/cm2) 1100-2000 25-640.

Example 8 Powders of Y2O3, BaCO3 and CuO were weighed 9.0791g, 31.7451g and 19.1858g, respectively, so that Y : Ba : Cu = 1 :
2 : 3 and placed in beaker, into which was poured 80ml of 60%
nitric acid aqueous solution for completel~ dissolving the powders to obtain a starting powder solution, to which were : : added 120g of citric acid and fully stirred for complete dissolution. Then, 28% aqueous ammonia was added for neutralization to thereby obtain a pale transparent (neutralized) solution of pH 7, which which was subsequently heated at 200C with the result after water was e~aporated, a porous mass was pyrolized and burnt, so that sponge material .~ :

13~3~

was obtained, which was confirmed by X-ray dif~raction to be a mixture of Y2O3, BaCO3 and CuO. The mixture ~as pulverized in an automatic mortar for 30 minutes to produce a powder of a particle size about 0.1 to~about 0.6 ~m, which was calcined for 900C for 24 hours in amhient flowing oxygen. The calcin~d powder was t~en further pulverized by a ball mill to produce a pulverized powder, which was pressed at 2.5 metric tons/cm2 to form a compact bar, which was in turn heated at 890C for 12 hours in an oxygen gas atmosphere. This series of pulverizing, pressing and heating operations was repeated ~hree times to obtain compact with 6.9 mm diameter, which subsequently underwent sheathing, rotary swaging, sheath removing, and final heating in the same conditions and manne~
as in Example 1 except that the final heating was carried out at 890C for 12 hours, followed by slow cooling. The composite wire after swaging and the core of the superconductor had sintered density of 82% and 91% or more, respectively. The ~uperconductor exhibited a critical temperature of 91 K and critical current density of about 11000 A/cm2 at 77K.

Example 9 With control of pH 7 to 8 by adding 28% aqueous ammonia, a precipitation was produced in the citric acid added solution which was prepared in the same conditions and manner as in Example 8 except that 70.954Bg of citric acid was added to thé
starting material dissolved solution. The precipitation was dried at- 150C and it was confirmed~by X-ray diffraction that it was a~mixture of~Y, Ba,~ Cu~and O. The ~mixture was subjected to calcining, pressing, intermediate sintering, sheathing, rotary swaging, sheath removing, ~final heating and slow cooling in the same manner and conditions as in ~xample 8.
The superconductor, thus produced, was equal in critical : :

~3~3~

temperature and critical current density to the superconductor of Example 8.

Example 10 A pulverized, cal~cined powder was prepared by the same conditions and manner except the powder mixture was calcined at 850C for 24 hours. It was obsexved by microscopy that the pulverized, calcined powder had an a~erage particle size of 5 ~m or smaller. The powder was pressed by a rubber press at 2.5 t/cm2 to form a rod compact, which was heated at 850C for 24 hours in an oxygen atmosphere and then gradually cooled at -200C/l~our to produce a rou~d rod intermediate ~inter, containing a superconductive oxide YlBa2Cu3O7_~, of which average particle size was confirmed to be 10 ~m or smaller.
The intermediate sinter was sheathed, underwent rotary swaging, and then~removal of-the silver sheath in a manner similar to the manner in Example 1 to produce an exposed core.
The exposed core was subsequently heated at 850C for 50 hours in an oxygen atmosphere and then gradually cooled to room temperature at a speed of -200C/hour to produce a superconductor. The superconductor exhibited a critical temperature of 91K and about 10,000 A/cm2 at 77K and had a density of 5.8 g/cm3 at its superconducting portion.

Comparative Test 13 ; An oxide superconductor was produced in the same condi~ions~and manner except that in each of the calcining, intermèdiate sintering and the final sintering! the heating temperature was 980C. This superconductor has a density of 5.~8 g~cm3 at is superconducting portion.

Exampl~ 11 1 3 ~ 2 A calcined powder was prepared in the same conditions and manner as in Example 1 and then heated at 890C for 14 hours in an oxygen atmosphere to produce YlBa2Cu3O7_~ superconductor, which was then placed in a~platinum crucible, where it was heated at 1300C in an oxygen atmosphere to melt. The molten material was rapidly c"ooled to 900C in an oxygen atmosphere and was maintained at this temperature for lO hours, after which it was gradually cooled to room temperature at -200C/hour to form a solidified material. A surface layer of the solidified material was taken away and was pulverized to produce a powder, which was pressed with a rubber press to form a rod compact having 8 mm diameter. This rod was shea~hed with a silver pipe having 1~ mm oute- diameter and 10 mm inner diameter to form a preform, which was diameter reduced by rotary swaging machine and drawing die for 1.O mm diameter composite wire. The silver sheath was removed by dissolving with a dilute nitric acid for exposing the core, which was then heated at 890C for 3 hours in an oxygen atmosphere to produce an oxide superconductor. This superconductor exhibited critical current density (Jc) of 1.6 x 104 Aicm2 in zero magnetic field and 1.2 x 104 A/cm2 in 2T
magnetic field.

Example 12 ;~ ~ Powders of Y2O3 with particle size 4 ~m, BaCO3 with ~particle~size 1 ~m, and~CuO with particle size 1 ~m were mixed with~a ball mill at mole ratio Y: Ba: Cu = 1 : 2 : 3 to obtain a powder mlxture~ which was calcined, pulverized and then pressed to~form a bar shaped~compact in thé same conditions and~manner~as in Example~3. This series of heating, pwlverizing and pressing operations were repeated to produce a 6.9 mm diameter compact, of which sintered density was about 78~ of the theoretical density with critical current density -of about 40 A/cm2. This compact was sheathed and underwent ~3~L3~3~

rotary swaging, removal of the sheath, the final heating and slow cooling in the same conditions and manner as in Example 3 except for forging ratio of 20% per each pass. The green density of the core of the composite wire after the rotary swaging was 82 % and the sintered density of the superconductive core ~as about 91.5 % of the theoretical density. The core was coated with 1 mm thick protective coating by solder coating. The superconductor thus produced exhibited a critical temperature of 91K and a critical current density of about 11,000 A/cm2 at 77K and also showed a sufficient mechanical strength in winding a magnetic core.

Example 13 The starting material powder of Example 1 was calcined at the same temperature for 12 hours and then pulverized to form a calcined powder, which was heated at 890C for 12 hours at ambient flowing oxygen of 2 liters~min. The heated powder was charged into a silver pipe having an inner diameter 7 mm and outer 7 mm with a 2 mm diameter silver core wire inserted into it to thereby obtain a preform, which was rotary swaged in a stepwise manner to have a diameter 1.4 mm with a forging ratio of about 10 % for each pass at a processing speed of 1 m/min.
The composite wire thus obtained underwent sheath removal, final heating, and slow cooling in the same conditions and manner as in Example 1 except that the slow cooling was carried out at -200C/hour to produce an oxide superconductor, which was then coated with a protection coating layer by solder plating to obtain a superconducting wire with an outer diameter 1 mm. The superconducting wire exhibited a critical temperature of 92K and a critical current density of about 12,000 A/cmZ at 77K. This superconductor could be wound around a magnet core without any substantial crack and showed a sufficient mechanical strength.

~3~3~32 Example 14 The starting material powder mixture of the Example 1 ~as calcined, pulveri~ed and then pressed at 2.5 metric tons/cm2 to form a rod compact in the same conditions and manner as in Example 1 except that"the pulverized, calcined powder was charged into a rubber tube with inner diameter 7 mm and outer diameter 10 mm, which was then placed within a vacu~m chamber held at a vacuum level about 10-4 mmHg. In this condition, the rubber tube was sealed and then pressed to form the rod compact, which was subsequently subjected to intermediate sintering, sheathing, rotary swaging, sheath removing, final heating and then slow coolin~ in the same conditions and manner as in Example 1 except that the rod compact underwent intermediate sintering for 12 hours in an oxygen atmosphere.
The superconductor, thus obtained, exhibited a critical temperature (Tc) of 91K an~d a critical current density (Jc) of about 11,000 A/cm2 at 77K.

Example 15 The starting material powder mixture was calcined at 700C for 24 hours and then calcined 900C for 29 hours to produce a calcined powder, which was then charged into a silver pipe having an inner diameter 7 mm and outer diameter 10 mm to form a preform, which was subjected to rotary swaging, sheath removing and final heating in the same conditions~and manner except that the preform was diameter reduced to a diameter of 1.4 mm. In the rotary swaging, the preform was cold forged by changing traveling direction for each passing. The composite wire, thus formed, had a core with a diameter 0.8 mm. The superconductive core wire produced was coated with a 1 mm thick protection coating by solder plating.
The superconductor was equal in critical temperature and ~3~3~3~ `

eritieal eurrent density to the supereonduetor of Example 1 and also showed suffieient meehanieal strength.

Example 16 A supereonduetive eore wire was prepared in the same eonditions and manner as in Example 1 exeept that after the rubber pressing, the rod compaet was heated at 900C for 24 hours in an oxygen atmosphere to produce a eompaet having a diameter 6.9 mm, of which sintered density was 78~ of the theoretical density. The compact was then subjected to sheathing, rotary swaging, sheath removing and final heating in the same conditions and manner as in Example 1 except that the final heating was made at 900C for 24 hours. The composite wire had a core of 82% green density of the theoretical density and the superconducting core had a sintered density of 91.5~ of the theoretical density after the final heating. The superconductive core was coated with a 1 mm proteetive coating by solder plating to produce a supereonduetor, whieh was equal in critical temperature and eritieal eurrent density to the supereonduetor of Example 1 and also showed a suffieient meehanieal strength in winding a magnet eore.

Example 17 :~ .
An oxide superconductor was produced in the same conditions and manner as in Example 2 except that the rod-shaped compact, formed by the hydrostatic pressing, had a diameter 7 mm and length of lOO mm, that the preform was diameter reduces so that the composite had a diameter 3 mm and length~of about 23~ m. The superconductor had a critical temperature of 91 K and critical current density of about 110000 AJem2. An exeellent eutting was made to the :

~. .

~3~3~32 superconductor by a diamond cutter with little cracks due to the cutting Example 18 A superconductin~ powder including Y1Ba2Cu307_~ (O < ~ 5 5) was charged into an aluminum pipe having 10 mm outer diameter and 6 mm inner diameter to form a preform, which was diameter reduced by rotary swaging in a stepwise manner to form a composite with 1.5 mm outer diameter, which was in turn immersed in 50% sodium hydroxide for dissolving the aluminum sheath to expose the core. The exposed core was heated at S00C for 5 hours in an oxygen atmosphere for producing an oxide superconductor, of which critical current density at 77K
is given in Table ~.
:
Comparative Tests 14~and 15 An aluminum sheathed composite wire was prepared in the same conditions and manner as in Example 18 and removed by dissolving with 50~ sulfuric acid to obtain an exposed core for Comparative Test 14. Another composite wire was prepared : for Comparative Test 15 in the same conditions and manner as in Example 18 except that a silver sheath of the same configuration was used. The silver sheath was removed with ~ 50% nitric acid aqueous solution to exposed its core. The two ~ exposed cores were heat treated in the same conditions as in Example 18 to produce superconductors, each of which had cr~tical current density at 77K as ~iven in Table 4.

~ Table 4 :~ ~ :: ; ::
Comparative Tests : Ex. 18 19 15 Critical current .

~313~3~

density ~x 103 A/cm~) 23 4-9 5.2 Example 19 ~, Powders of Y203 (purity: 99.99%), BaC03 (purity: 99.9~) ~
and CuO (purity: 99.9~) were weighed so that Y : Ba : Cu = 1 :
2 : 3 in mole ratio, and then mixed to obtain a powder mixture, which were calcined at 900C for 24 hours in the atmosphere and then pulverized to produce a superconducting powder which includes an oxide superconductor Y1Ba2Cu3O7_~, The superconducting powder was pressed by a rubber press into a rod compact, which was heated at 890C for 12 hours in an~ient flowing oxygen gas of 2 liters/min to produce a sintered compact, which was in turn inserted into a copper pipe, having an inner diameter 8 mm and outer diameter 15 mm to form a preform. Subsequently, the preform was dawn into a composite wire having an outer diameter 1.5 mm and length 500 rnm, of which core had a diameter 0.8 mm. The composite wire was wound around a reel and introduced at a speed of 20 mm/min into a heating tube as shown in FIG. 8, where it was induction heated to melt the sheath for exposing the core. The heating tube had five high frequency induction coils of which first one had a length L1 of O.S m and the others had a length L2 of 3 m. Each coil was supplied with 30 kHz to 100 kHz of alternating current. Thus, the coils were adjusted so that ~he Pirst coil with 0.5~m length was provided with an output of 50 kW for enabling to melt the copper layer, which coated the core of the~composite, and the other coils were pro~ided with outputs of 20-100 kW Xor heating the exposed core of the composite at a temperature 890C *5C. In the slow cooling portion 42B of the heating tube had a length L3 5 m for gradually cooling the heated core. In the induction heating, the inside of the heating tube was placqd in an oxygen atmosphere by introducing hot oxygen gas at a flow rate of 2 liters/min via oxygen supply tubes 36. A receptacle 44 of FIG. 9 was located below the heating tube 40 for recovering copper molten from the composite 6. After the heat treatment, the heated core was introduced through the bottom of the treating bath E and then issued *rom the top thereof. During moving in the bath E, the core passed through molten Sn-Pb solder during which u~trasonic waves were applied to it with frequency 60 kHz and output lOW. After issuing from the bath, the core was cooled so that a superconductor wire coated with about 50 ~m solder coating was produced. No breaking of the superconductor wire was noted. The core of the superconductor had a critical temperature of 91.0C and critical current density of about 15000 A/cm2 in liquid nitrogen.

Example 20 An oxide superconductor was produced in the same manner and conditions as in Example 12: the starting material powder mixture was calcined in ambient flowing oxygen of l liter/min;
and the final heating was made also in ambient flowing oxygen of the same flow rate. This superconductor was equal in critical temperature and critical current density to the superconductor of Example 12.

Example 21 ThP starting material powder mixture was calcined, pulverized, pressed and heated in the same manner and conditions as in Example 1 except that the compact was heated at 890C for 14 hours in an oxygen atmosphere. The intermediate sinter, thus obtained, was sheathed with a silver pipe having a 10 mm outer diameter and a thickness 1.5 mm and then subjected to rotary swaging to form a composite wire with a 1.0 mm diameter, which was placed in a 50 % nitric acid aqueous solution for removing the silver sheath to expose the ~: .
, :

~3~3~32 core. The exposed core was heated at 890C ~or 12 hours in an oxygen atmosphere and then gradually cooled to produce an superconducting core, which was then coated with about 13 to about 20 ~m aluminum coatihg by placi~g it within an aluminum bath which was being vibrated with 20W 60kHz ultrasonic wave generator. Fifty supe,~conducting core wires of this example were prepared and pulled to pass through the first and second separators, as in FIG. 14, which were rotated at a low speed for twisting, into an aluminum bath with an ultrasonic wave generator and then to issue from it for solidifying the molten aluminum adhered to the core wire to form a multifilamentary superconducto~ with an aluminum stabili~er, which exhibited a critical temp~rature (Tc) of 91K and a critical curren~
density (Jc) of about 11,000 A/cm2 at 77K.

Example 22 BaCO3 and CuO powders, having a particle size about 3 ~m, were mixed so that Ba:Cu=2:3 at mole ratio, the mixture was then calcined at 880C for 10 hours in atmospheric air to produce a calcined powder, having a composition of Ba2Cu3Os.
The calcined powder was pulverized to a particle size about 10 ~m and then mixed with both Tl2O3 and CaO powders, having a particle size about 3 ~m to form a mixture so that Tl:Ca:Ba:Cu=2:2:2:3 at mole ratio. The starting material thus prepard was pressed to form a compact, with a green density 75~ of the theoretical density, which was then heated at 870C
for 1 hour in ambient Plowing oxygen 2 litexs/min, followed by slow cooling at -200C/hour to thereby produce an intermediate s1nter having a composition Tl2CazBa2Cu3O; (; undetermined) nd a~sintered density of about 8S% of the theroretical density.
The intermediate sinter was inserted into a silver pipe having an outer diameter 10 mm and a thickness 1.5 mm to form a preform, which was diameter reduced by a rotary swaging machine to a 0.5 mm diameter composite wire, which was then .~, . .

~3~3~2 immersed in a dilute nitric acid to remove the silver sheath for exposing the core, which was in turn heated at 870C for 30 minutes in ambient flowing oxygen gas of 2 liters/min to thereby produce a superconductor, having a composition Tl2Ca2Ba2Cu3Oj (j undetermined) and a sintered density about 92% of the theoretical density, which had a critical temperature of 120K and critical current density of 2 x 10 A/cm2 at 77K.

Example 23 Solutions of nitrates of Bi, Pb, Sr, Ca and Cu were mixed so that Bi: Pb: Sr:Ca:Cu=1.4:0.6:?:2:3 at mole ratio, and then ammonium oxalate was added to coprecipitate oxalates of the superconductor materials, which were dried to obtain a powder mixture with a particle size about O.l~m, which was in turn calcined at 820C for 12 hours in atmospheric air to produce a calcined powder. The calcined powder was charged into a silver pipe, having an outer diameter 10 mm and a thickness 1.5 mm, to form a preform, which was then diameter reduced by a rotary swaging machine to form a 1.5 mm diameter composite wire, having a 0.8 mm diameter core of a green density 85% of the theoretical density, which was then passed through high frequency induction coils to remove the silver sheath for exposing the core. Subsequently, the exposed core was heat treated at 850C for 50 hours in atmospheric air to thereby produce a superconductor having a composition Bi2PbuSr2Ca2Cu3Ov(u and v undetermined) and a sintered denslty of about 95% of the theoretical density, which was then coated with a 1 mm thick ceramics solder protection coating in a solder ba~h, containing a molten ceramics solder which includes lead, zinc, tin, aluminum, antimony, titanium, silicon, copper and cadmium, during application of 60 k~z ultrasonic waves at lOW output to the surface of superconductor. The coated superconductor had a critical ....

~3~ 3~3~

temperature of 105K and critical current density of 1 x 104 A/cm2 at 77K.

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Claims

1. A method of producing a superconducting wire including a superconductive oxide represented by the formula AxByCzD7-.delta.
provided that the A is at least one selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, the B is at least one selected from the group consisting of Be, Sr, Mg, Ca, Ba and Ra, the C includes Cu, the D includes O, about 0.1 < x < about 2.0, about 1 < y <
about 3, about 1 < z < about 3, 0 < .delta. < 5, or by the formula AxByCazCuiOj provided that the A includes Bi or T1, the B includes Sr or Ba, about 1 < x < about. 3, about 1 < y < about 3, about O < z < about 3 and about O < i < about 4, comprising the steps of:
(a) charging into a metallic pipe for forming a preform at least one filling material selected from the group consisting of: a starting material powder of the superconductive oxide; a powder of the superconductive oxide;
and a compact, made of the starting material powder and/or the superconductive oxide powder;
(b) reducing in cross-sectional area the preform for forming a composite including a core, made of the at least one filling material, and a metallic sheath covering the core;
(c) removing the metallic sheath of the composite, cross-sectional area reduced in the step (b), for exposing the core;
and (d) heat treating the exposed core for producing the superconductive oxide.

2. A method as recited in Claim 1, wherein the superconductive oxide is represented by the formula AxByCazCuiOj where the A is Tl, the B is Ba, x = 2, y = 2, z = 1 or 2 and i = 2, 3 or 4.

3. A method as recited in Claim 1, wherein the superconductive oxide is represented by the formula AxBycazcuioi where the A is Bi, the B is Sr, x = 2, y = 2, z = 1 or 2 and i = 2 or 3.

4. A method as recited in Claim 1, wherein the metallic sheath removing step (c) comprises the steps of dipping the metallic sheath in a treating liquid for melting the metallic sheath to remove from the core and then taking out the metallic sheath removed core from the treating liquid.

5. A method as recited in Claim 4, after the taking out step, further comprising the step of preventing the treating liquid, adhered to the core, from adversely affecting the core.

6. A method as recited in Claim 5, wherein the metallic pipe is made of an alkali soluble metal selected from the group consisting of aluminum, iron and their alloy, and wherein the treating liquid is an aqueous solution of an alkali which dissolve the alkali soluble metal.

7. A method as recited in Claim 6, wherein the adverse affection preventing step comprises the steps of neutralizing the the aIkali aqueous solution adhered to the core, and after the neutralization step, further comprising the step of coating the core with a protection layer for stabilizing the superconductor in performance.

8. A method as recited in Claim 1, wherein the metallic sheath removing step (c) comprises the step of machining the metallic sheath for removing thereof.

9. A method as recited in Claim 1, wherein the metallic sheath removing step (c) comprises the step of high frequency induction heating the composite for melting the metallic sheath to thereby remove the metallic sheath.

10. A method as,recited in Claim 9, wherein the heat treating step (d) comprises high frequency induction heating the exposed core for producing the oxide superconductor.

11. A method as recited in Claim 10, wherein the metallic sheath removing step comprises the step of preparing a high frequency induction heating coil, further comprising the step of energizing the induction heating coil, and wherein the metallic sheath removing step and the heat treating step are carried out by passing the core through the induction heating coil.

12. A method as recited in Claim 11, wherein the A is Y, the B is Ba, x=1, y=2, z=3 and .delta.=about 0, and wherein the heat treating step comprises the step of gradually cooling the core, in which the oxide superconductor has produced, for transforming a crystal structure of the superconductor from a cubic system to a orthorhombic system.

13. A method as recited in Claim 12, wherein the induction heating coil preparing step comprises the step of holding the induction coil with an axis thereof vertically arranged, and wherein the core vertically passes through the the induction coil.

14. A method as recited in Claim 1, wherein the heat treating step (d) is carried out at about 800 to about 1000°C
for about 1 to about 500 hours.

15. A method as recited in Claim 14, wherein the heating step (d) is carried out for forming a spherical crystal structure of the oxide superconductor.

16. A method as recited in Claim 15, wherein the heating step (d) is carried out at about 850 to 920 °C for about 1 to about 100 hours.

17. A method as recited in Claim 1, after the heating step (d), further comprising the steps of applying ultrasonic waves to the core for improving wettability thereof and forming a protection coating over the core while the ultrasonic waves are applied.

18. A method as recited in Claim 17, wherein in the ultrasonic waves applying step, the ultrasonic waves have a frequency of about 5 to about 200 kHz.

19. A method as recited in Claim 1, after the heating step (d), further comprising the steps of: preparing a plurality of cores heat treated in the step (d); bundling the prepared cores into a core bundle; passing the core bundle through a molten metal for adhering the molten metal to each of the cores of the core bundle, the molten low melting point metal has a melting point below a temperature at which the heat treatment is carried out; and cooling the molten metal adhered core bundle for burying the core bundle in the cooled molten metal to thereby form a multicore superconducting wire.

20. A method as recited in Claim 19, wherein the low melting point metal is a metal selected from the group of consisting of aluminum, tin, zinc, indium, gallium, lead, and bismuth.

21. An oxide superconductor produced by the method of one of Claims 1 to 20.